Rechargeable batteries are used in a wide variety of applications, for example as a propulsion power source for electric and hybrid vehicles or as a power reserve in telecommunications network stations. In any application, it is important to monitor the discharge capacity of the battery at any given time, as well as to monitor the state of health of the battery as it ages through degradation of its chemistry over repeated floating/charging/discharging cycles. This information helps to manage the timely replacement of a battery approaching the end of its useful life.
A battery can be approximated as a voltage source with an internal resistance. The internal resistance of a battery varies with battery age, but remains relatively constant over a short time period. As the battery ages during floating, charging and discharging, its internal resistance increases. The increase of the internal resistance is caused by the degradation of the battery's chemistry, which in turn reduces the battery's ability to hold a charge. The performance of a battery is characterized by its discharge curve, which is a curve of the battery voltage as a function of time at a predetermined discharge rate or as a function of the percentage of the remaining charge of the battery. As the internal resistance of the battery increases, the discharge curve drops, indicating a reduction of the battery capacity. The battery discharge curve may vary with the internal resistance of the battery, its discharge rate and its temperature.
Various systems, methods and apparatus have been devised to estimate the battery capacity and the battery remaining life based on the internal resistance of the battery. U.S. Pat. No. 5,404,106 discloses an electronic control unit, switching means for connecting and disconnecting the positive and negative terminals of the battery from an electrical load, first and second voltage measurement means for measuring battery voltage in the connected and disconnected states, discharge current measurement means and battery electrolyte temperature sensing means. The voltage and current measurements are relayed to the control unit, which calculates the internal resistance of the battery. The remaining capacity level of the battery is then estimated from stored values corresponding to the calculated internal resistance, and a correction coefficient associated with the electrolyte temperature reading is applied to the remaining capacity level of the battery to more accurately estimate the remaining battery capacity.
U.S. Pat. No. 6,087,808 discloses a system and circuit means that measure the internal resistance of the battery and an output current at varying battery loads. These measurements are relayed to a computer having stored in its memory an array of discharge curves specific to the battery type. Software running on the computer employs the internal resistance and output current readings to select from its memory a discharge curve most closely associated with the state of the battery as characterized by the internal resistance and output current values. The selected discharge curve is then used to estimate the relative remaining life of the battery.
U.S. Pat. No. 6,167,309 discloses a system or process for estimating the level of power depletion in a cardiac pacing system having a lithium battery, by measuring and correlating the rate of charge of a charge storage capacitor connected to the lithium battery to a value of the internal resistance of the battery, which provides an estimation of the remaining battery capacity and the replacement time for the battery.
Each of the systems or methods described above provide an estimation of the remaining battery capacity based on a correlation of voltage measurements taken at the battery terminals to a discharge curve. Unfortunately, these voltage measurements prove inaccurate when a battery discharge curve comprises a mild slope and are inadequate when a battery discharge curve comprises a plateau wherein the voltage of the battery is not sufficient to establish the remaining battery capacity. The systems or methods described above also measure an output current to determine the internal resistance of the battery. However, the measurement of an output current to determine the internal resistance renders the value of the internal resistance somewhat approximate. Also, the internal resistance is calculated based on measurements taken at the battery terminals, and provides little information on the state of the individual cells of the battery.
Furthermore, none of the systems or methods described above provide an accurate evaluation of the state of health of a battery, in order to predict how long the battery may be able to perform before reaching a level of performance (battery capacity) at which replacement is required.
Against this background, it clearly appears that a need exists in the industry for an improved system for accurately predicting the remaining capacity of an energy storage device, as well as for accurately evaluating the state of health of the energy storage device.